Exploring Jets from a Supermassive Black Hole

What are the feeding — and burping — habits of the supermassive black holes peppering the universe? In a new study, observations of one such monster reveal more about the behavior of its powerful jets.

Beams from Behemoths

Across the universe, supermassive black holes of millions to billions of solar masses lie at the centers of galaxies, gobbling up surrounding material. But not all of the gas and dust that spirals in toward a black hole is ultimately swallowed! A large fraction of it can instead be flung out into space again, in the form of enormous, powerful jets that extend for thousands or even millions of light-years in opposite directions.

M87

M87, shown in this Hubble image, is a classic example of a nearby (55 million light-years distant) supermassive black hole with a visible, collimated jet. Its counter-jet isn’t seen because relativistic effects make the receding jet appear less bright. [The Hubble Heritage Team (STScI/AURA) and NASA/ESA]

What causes these outflows to be tightly beamed — collimated — in the form of jets, rather than sprayed out in all directions? Does the pressure of the ambient medium — the surrounding gas and dust that the jet is injected into — play an important role? In what regions do these jets accelerate and decelerate? There are many open questions that scientists hope to understand by studying some of the active black holes with jets that live closest to us.

Eyes on a Nearby Giant

In a new study led by Satomi Nakahara (The Graduate University for Advanced Studies in Japan), a team of scientists has used multifrequency Very Long Baseline Array (VLBA) and Very Long Array (VLA) images to explore jets emitted from a galaxy just 100 million light-years away: NGC 4261.

This galaxy’s (relatively) close distance — as well as the fact that we’re viewing it largely from the side, so we can clearly see both of its polar jets — allows us to observe in detail the structure and intensity of its jets as a function of their distance from the black hole. Nakahara and collaborators’ observations span the enormous radial distance of a thousand to a billion times the radius of the black hole, or about 54 light-days to more than a million light-years.

Scale for Change

width vs. radius of jet

The width of the jet as a function of radial distance from the black hole, for NGC 4261 (red) compared to the few other jets from nearby supermassive black holes that we’ve measured. NGC 4261’s jets transition from parabolic to conical at around 10,000 times the radius of the black hole (RS). [Nakahara et al. 2018]

The authors’ observations of NGC 4261’s jets indicate that a transition occurs at ~10,000 times the radius of the black hole (that’s a little over a light-year from the black hole). At this point, the jets’ structures change from parabolic (becoming more tightly beamed) to conical (expanding freely). Around the same location, Nakahara and collaborators also see the radiation profile of one of the jets change, suggesting the physical conditions in the jets transition here as well.

This is the first time we’ve been able to examine jet width this closely for both of the jets emitted from a supermassive black hole. The fact that the structure changes at the same distance for both jets indicates that the shape of these powerful streams is likely governed by global properties of the environment surrounding the galaxy’s nucleus, or properties of the jets themselves, rather than by a local condition.

The authors next hope to pin down velocities inside NGC 4261’s jets to determine where the jets accelerate and decelerate. This nearby powerhouse is clearly going to be a useful laboratory in the future, helping to unveil the secrets of more distant, feeding monsters.

Bonus

Curious what these hungry supermassive black holes look like? Check out this artist’s imagining of NGC 4261, which shows how it feeds from a large, swirling accretion disk and emits fast-moving, collimated jets. [Original video credit to Dana Berry, Space Telescope Science Institute]

Citation

Satomi Nakahara et al 2018 ApJ 854 148. doi:10.3847/1538-4357/aaa45e

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